Some hearing implants such as Middle Ear Implants (MEI's) and Cochlear Implants (CI's) employ cooperating attachment magnets located in the implant and the external part to magnetically hold the external part in place over the implant. For example, as shown in FIG. 1, a typical cochlear implant system may include an external transmitter housing 101 containing transmitting coils 102 and an external attachment magnet 103. The external attachment magnet 103 has a conventional cylindrical disc-shape and a north-south magnetic dipole having an axis that is perpendicular to the skin of the patient to produce external magnetic field lines 104 as shown. Implanted under the patient's skin is a corresponding receiver assembly 105 having similar receiving coils 106 and an implant magnet 107. The implant magnet 107 also has a cylindrical disc-shape and a north-south magnetic dipole having a magnetic axis that is perpendicular to the skin of the patient to produce internal magnetic field lines 108 as shown. The internal receiver housing 105 is surgically implanted and fixed in place within the patient's body. The external transmitter housing 101 is placed in proper position over the skin covering the internal receiver assembly 105 and held in place by interaction between the internal magnetic field lines 108 and the external magnetic field lines 104. Rf signals from the transmitter coils 102 couple data and/or power to the receiving coil 106 which is in communication with an implanted processor module (not shown).
One problem arises when the patient undergoes Magnetic Resonance Imaging (MRI) examination. Interactions occur between the implant magnet and the applied external magnetic field for the MRI. As shown in FIG. 2, the direction magnetization  of the implant magnet 202 is essentially perpendicular to the skin of the patient. In this example, the strong static magnetic field  from the MRI creates a torque  on the internal magnet 202, which may displace the internal magnet 202 or the whole implant housing 201 out of proper position. Among other things, this may damage the adjacent tissue in the patient. In addition, the external magnetic field  from the MRI may reduce or remove the magnetization  of the implant magnet 202 so that it may no longer be strong enough to hold the external transmitter housing in proper position. The implant magnet 202 may also cause imaging artifacts in the MRI image, there may be induced voltages in the receiving coil, and hearing artifacts due to the interaction of the external magnetic field  of the MRI with the implanted device. Torque and forces acting on the implant magnet and demagnetization of the implant magnet are especially an issue with MRI field strengths exceeding 1.5 Tesla.
Thus, for existing implant systems with magnet arrangements, it is common to either not permit MRI or at most limit use of MRI to lower field strengths. Other existing solutions include use of a surgically removable magnets, spherical implant magnets (e.g. U.S. Pat. No. 7,566,296), and various ring magnet designs (e.g., U.S. Provisional Patent 61/227,632, filed Jul. 22, 2009). U.S. Patent Publication 20110264172 describes an implant magnet having a magnetic dipole with a magnetic axis that is parallel to the end surfaces of a disc shaped implant magnet—that is, perpendicular to the conventional magnetic axis of a disc-shaped implant magnet. The magnet is then held in a magnet receptacle that allows the magnet to rotate in response to an external magnetic field such as from an MRI.
Some devices also add a stiffening ring around the magnet to resist torques and help hold the magnet in place. FIG. 3 shows an example of a cochlear implant device 300 with an implantable stimulator 301 that provides electrical stimulation signals to an electrode lead 302 that is implanted in the patient's cochlea. A coil case 303 is made of biocompatible resilient material such as molded silicone in which is embedded a communications coil 304 for transcutaneous communication of an implant communication signal. In the center of coil case 303 is an implant magnet 306 that cooperates with another external holding magnet (not shown) to hold an external coil on the skin of the patient over the implanted communications coil 304. Also embedded in the resilient material of the coil case 303 between the communications coil 304 and the implant magnet 306 is a stiffening ring 305 made of stiffer material than the coil case 303. The stiffening ring 305 is configured to resist mechanical torque movement of the coil case 303 and to promote securement of the implant magnet 306 within the coil case 303. This includes securement of the implant magnet 306 against movement and tilting, and in the case of a removable implant magnet 306, additionally against magnet displacement in lateral direction (i.e. perpendicular to the skin surface).